ci: Travis: branches: only build master branch

Ref: vim-jp#137 (comment)

Author: blueyed

.travis.yml

@@ -59,3 +59,7 @@ after_success:
   - coverage report -m
   - coverage xml
   - bash /tmp/codecov.sh -f coverage.xml -F vimscript
+
+branches:
+  only:
+    - master